Broad molecular weight distribution (MWD) low density polyethylenes (LDPEs) with high molecular weight fractions are desired for good processability (line speed, bubble stability, neck-in, etc.) in pure form, or in blends with linear low density polyethylene (LLDPE) or other polymers. Broad MWD LDPE is made up a range of polymer molecules, including low and high molecular weight polymer molecules. The extractable fraction in such polymers generally increases with an increasing fraction of low molecular weight molecules, and also increases by increasing the short chain branching frequency in the low molecular weight molecules. There is typically a trade-off between the broadness of the MWD and the extractable level. The broad MWD can aid in areas, such as processability of the LDPE during processing operations such as extrusion with lower pressures or better bubble stability when making a blown film. The extractable level is an important parameter for food packaging applications, and low extractables are desirable, since high extractable levels typically lead to smoke formation and/or die build-up during processing of the LDPE. Additionally, the LDPE may be used in food contact applications, and if the extractable levels are too high, the LDPE will not meet Food and Drug Administration (FDA) limits for extractables for non-cook-in and cook-in applications, thereby restricting the use of the LDPE from some applications.
Broad MWD LDPEs can be made in different reactor types, such as autoclave or tube reactors with different distributions of residence times. Due to the broader residence time distribution of an autoclave reactor, it is much easier to make a broad MWD with an ultra high molecular weight polymer fraction in this type of reactor. The broad MWD of an autoclave product, however, does result in a high molecular weight fraction which may adversely affect properties, such as film optics. Broad MWD LDPEs are more difficult to achieve in a tubular reactor due to the plug flow behavior. More extreme process conditions, such as high temperature, low pressure, and/or higher conversion level, etc., have to be applied to obtain a broad MWD in a tubular reactor. These extreme process conditions typically lead to more extractables; however the MWD is free of extreme high molecular weight fractions as seen in broad MWD autoclave-based polymers. There is a need to reduce the extractable levels in very broad MWD tubular products by changing the extractability of the low molecular weight fraction in the polymer, as evidenced by the analysis of the extracted low molecular weight fraction.
International Publication No. WO 2007/110127 discloses an extrusion coating composition comprising an ethylene copolymer. The ethylene copolymer is obtained by a polymerization that takes place in a tubular reactor at a peak temperature between 300° C. and 350° C.
International Publication No. WO 2006/094723 discloses a process for the preparation of a copolymer of ethylene and a monomer copolymerizable therewith. The polymerization takes place in a tubular reactor at a peak temperature between 290° C. and 350° C. The comonomer is a di- or higher functional (meth)acrylate, and the comonomer is used in an amount between 0.008 mol % and 0.200 mol %, relative to the amount of ethylene copolymer. The di- or higher functional (meth)acrylate is capable of acting as a crosslinking agent.
European Patent EP 0928797B1 discloses an ethylene homo or copolymer having a density of between 0.923 and 0.935 g/cc, and a molecular weight distribution Mw/Mn between 3 and 10, and comprising from 0.10 to 0.50 wt % of units derived from a carbonyl group containing compound, based on the total weight of the homopolymer or copolymer.
U.S. Pat. No. 3,334,081 discloses a continuous process for the production of polymers of ethylene as carried out in a tubular reactor, whereby the polymer is obtained at a higher conversion rate. In one embodiment, this patent discloses a continuous process for the polymerization of ethylene in a tubular reactor at a pressure of at least about 15,000 psig, and a temperature from about 90° C. to about 350° C., in the presence of a free radical initiator.
U.S. Pat. No. 3,657,212 discloses the production of ethylene homopolymers having a specific density, by polymerization of ethylene, under the action of organic peroxides and oxygen as free-radical-generating polymerization initiators, and of polymerization modifiers, at elevated temperature and superatmospheric pressure, in a tubular reactor having two successive reaction zones. A mixture of ethylene, polymerization initiator, and polymerization modifier are introduced continuously at the beginning of each reaction zone. The ethylene homopolymers have a broad molecular weight distribution, but are said to be practically devoid of very high molecular weight constituents.
Additional polymerizations and/or resins are described in the following: U.S. Pat. Nos. 2,153,553; 2,897,183; 2,396,791; 3,917,577; 4,287,262; 6,569,962; 6,844,408; 6,949,611; U.S. Publication Nos. 2007/0225445; 2003/0114607; US2009/0234082; International Publication Nos. WO 2012/044504; WO 2011/075465; WO 2008/112373; WO 2006/096504; WO 2007/110127; GB1101763; GB1196183; DD120200; DD276598A3; DE2107945; EP0069806A1; CA2541180; EP1777238B1; EP0792318B1; EP2123707A1; and J. Bosch, “The Introduction of Tubular LDPE to the Extrusion Coating Market and the Specifics of the Product,” 12th TAPPI European PLACE conference, 2009, pages 1-20.
Two-zone tubular reactor systems, as commonly used in the above art, lead to polymers with either too narrow MWD or too high extractable levels. Achieving broad MWD resins with such reactor systems typically requires extremely high peak temperatures and/or low reactor inlet pressures, and both lead to the formation of lower molecular weight material with increased short chain branching level, which leads to high extractables. Thus, conventional tubular polymerization processes can produce relatively broad MWD polymers, but with high levels of extractables. As discussed above, there remains a need to reduce extractable levels in very broad MWD tubular products. These needs and others have been met by the following invention.